1. Field of the Invention
The present invention relates to a voltage nonlinear resistor for use for stabilization of circuit voltages in various kinds of electronic appliances, for absorbing surge, noise, etc., and particularly relates to a voltage nonlinear resistor in which the leakage current after the resistor is soldered to a substrate is reduced without changing the varistor function from that of a conventional one.
2. Description of the Related Art
Zinc oxide varistors have been heretofore widely used in household electric appliances, and so on, because of its excellent nonlinear characteristic and its excellent energy characteristic. The zinc oxide varistors have been also widely used for other purposes recently because attention has been paid to the use of zinc oxide varistors as laminated chip varistors of the type embedded in a surface as parts for countermeasures to noise and static electricity. Further, their terminal electrodes have been plated in order to improve solderability.
In the existing circumstances, although the leakage current at the time of application of a circuit voltage to each a single part has caused no problem, but there was a problem in that the leakage current at the time of application of a circuit voltage after soldering to a substrate has increased. For example, a device actuated by a battery had a disadvantage in that the operating time was shortened.
This was because a portion of low resistance was generated in a surface of the laminated chip varistor when soldering was carried out. A method of coating a surface of a base with glass or epoxy resin (JP-A-5-129104) is known as a countermeasure to this problem.
It is also known that the leakage current after soldering is reduced by reducing the amount of an additive such as aluminum added to a varistor composition. In this case, however, the nonlinear index (xcex1) as a varistor characteristic was so small that the varistor could not be generally used as a surge and noise absorptive protection device easily.
In the aforementioned method of coating a surface of a base with glass or epoxy resin in order to reduce the leakage current after soldering, work was complicated, and the reduction in yield, the failure in soldering, or the like, was caused because of coating of unnecessary portions. There was, therefore, a problem in lowering of reliability and in increase of cost.
Further, when leakage current was to be reduced due to the amount of addition of aluminum oxide, the nonlinear index (xcex1) could be set at a sufficient value by increasing the amount of addition of aluminum oxide, but, in this case, there was a problem that the leakage current was increased conversely.
Accordingly, an object of the present invention is to provide a laminated chip type varistor improved in the aforementioned problems.
To achieve the foregoing object, the present invention provides laminated chip type varistors having the following configurations.
A laminated chip type varistor including a varistor function layer, internal electrodes, and terminal electrodes, wherein: the varistor function layer has a composition containing zinc oxide as a main component, and cobalt oxide and rare earth elements as additives; and the internal electrodes contain at least one member selected from the group consisting of aluminum in the form of Al2O3 with an amount of from 0.0001 to 5.0% by weight, iron in the form of Fe2O3 with an amount of from 0.0001 to 5.0% by weight, and zirconia in the form of ZrO2 with an amount of from 0.001 to 6.0% by weight as additives with respect to an electrically conductive metal component of a composition for forming layers of the internal electrodes.
It is more preferable that a laminated chip type varistor comprising a varistor function layer, internal electrodes, and terminal electrodes, wherein: the varistor function layer has a composition containing zinc oxide as a main component, and cobalt oxide and rare earth elements as additives; and the internal electrodes contain at least one member selected from the group consisting of aluminum in the form of Al2O3 with an amount of from 0.0001 to 0.5% by weight, iron in the form of Fe2O3 with an amount of from 0.0001 to 0.5% by weight, and zirconia in the form of ZrO2 with an amount of from 0.001 to 0.5% by weight as additives with respect to an electrically conductive metal component of a composition for forming layers of the internal electrodes.
With such a configuration, laminated chip type varistors having the following effects can be provided.
(1) The leakage current after soldering can be a small value which is not larger than 7.5 xcexcA without lowering the varistor function.
(2) The leakage current after soldering can be a further smaller value which is not larger than 7 xcexcA without lowering the varistor function.